U.S. Pat. No. 3,842,194 of Clemens discloses a capacitive information disc record having a playback system utilizing variable capacitance. In one configuration of the Clemens system, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths of about 2.6 micrometers and groove depths of about 0.5 micrometer may be used. During playback a pickup stylus of about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick engages the groove as the record is rotating by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the pre-recorded information.
In systems of the above type, the use of a relatively fine record groove and the record engagement requirements of the pickup stylus result in a stylus tip which is extremely small.
In U.S. Pat. No. 4,162,510 of Keizer, a novel keel-tipped pickup stylus structure is disclosed. The keel-tipped pickup stylus comprises a dielectric support element having a body, a constricted terminal portion, and shoulders interconnecting the body with the constricted terminal portion. The electrode is remote from the keel tip.
A second patent of Keizer, U.S. Pat. No. 4,104,832, discloses a pyramidal dielectric support element which is shaped on an abrasive lapping disc having a deep, coarse-pitched groove in order to obtain a keel-tipped stylus. Glow discharge deposited SiO.sub.2 is used by Keizer as an abrasive coating. The coating is prepared by a method which utilizes as starting materials oxygen and an alkoxy-substituted silane of the formula ##STR1## wherein R.sub.1 is selected from the group consisting of H and CH.sub.3, R.sub.2 and R.sub.3 are independently selected from the group consisting of H, CH.sub.3, OCH.sub.3, and OC.sub.2 H.sub.5, and R.sub.4 is selected from the group consisting of OCH.sub.3 and OC.sub.2 H.sub.5.
Coatings employed in Keizer require periods as long as 30 minutes to shape one diamond stylus. Furthermore, these coatings quickly loose their abrading ability and a second diamond stylus may require two hours to be lapped.
Kaganowicz is a copending application entitled, "METHOD FOR PREPARING AN ABRASIVE COATING", Ser. No. 963,819, filed Nov. 27, 1978, discloses a method for preparing an abrasive silicon oxide (SiO.sub.x, 1.ltoreq.x.ltoreq.2) coating on the substrate comprising glow discharging precursors comprising silane and a gaseous, oxygen-containing compound selected from the group consisting of N.sub.2 O, H.sub.2 O and CO.sub.2.
In preparing a lapping disc by glow discharge deposition methods a plastic disc is often employed as the substrate. Because a considerable amount of heat is generated during the glow discharge process, the glow discharge deposition must be interrupted to prevent the plastic substrate disc from warping. These interruptions lead to greater time requirements for preparing an abrasive lapping disc. It would therefore be desirable to have a method which allows for the preparation of an abrasive SiO.sub.x coating of suitable thickness without frequent interruption.
Wang et al. in a copending application entitled, "METHOD OF DEPOSITING AN ABRASIVE LAYER", Ser. No. 048,161, filed June 13, 1979, now U.S. Pat. No. 4,260,647 teach a method of depositing an SiO.sub.x layer onto a substrate by depositing a series of thin layers by glow discharge of an organosilane and oxygen. After each interruption of the deposition, a glow discharge is initiated in oxygen prior to a subsequent SiO.sub.x deposition. Because of the vigorous spontaneous interaction between O.sub.2 and SiH.sub.4 the Wang et al. method is not attractive for the present problem.